Earth Science Standards 3.a - Students know features of the ocean floor (magnetic patterns, age, and sea-floor topography) provide evidence of plate tectonics.3.b - Students know the principal structures that form at the three

different kinds of plate boundaries.

Plate Tectonics and

Plate Boundaries

Wegener’s Evidence

• Matching Coastlines• Rock Evidence

• Fossil Evidence• Climate Change

Evidence

Hess’s Evidence

Two Types of Crust1. Oceanic Crust• Mainly Basalt (more dense rock – sinks under

continental crust)• Thickness: 3-5 miles (thinnest at the ocean

ridges)• Mineral composition: low in silicates; dark-

colored rock (high in magnesium and iron)

2. Continental Crust• Mainly Granite (less dense rock – floats on top of

the mantle)• Thickness: 18-30 miles• Mineral composition: lots of silicates; light-

colored rock (quartz and feldspar)

• The lithosphere (crust and solid upper mantle) of Earth is broken into fragments called plates

• Vary from 62-125 mi in thickness

• Plates move on top of the hot asthenosphere

Earth’s Plates

• There are about 8 major plates and nine minor ones.

• The major plates include both continents and oceans.

• The number of plates and their sizes, shapes, and motions have changed throughout Earth's history ... and continue to do so today.

• Different plates move at different speeds.

What plate are we

riding on?

• Because continental crust is less dense and thicker than oceanic crust, it "floats" higher on the underlying mantle rock.

• Oceanic crust is made of dense basalt; continental crust consists of less dense granite.

Why do continents rise above the ocean floor?

Notice how high the continental crust rises above the oceanic crust in this cross section of the North American Plate.

• Plate Boundaries – places where 2 broken plates meet

• Most earthquakes, volcanoes, and mountains occur at plate boundaries

Plate Boundaries

Plotting earthquakes

and volcanoes help scientists

define plate boundaries!

3 Types of Plate Boundaries1. Divergent – Separation2. Convergent - Collision3. Transform – Sliding Past

• Plates move apart• Mostly on seafloor but

can occur on land• Occurs when 2 oceanic

plates separate or when 2 continental plates separate

• Hot, rising material causes the plates to separate (convection currents)

1. Divergent

• On seafloor:• Oceanic crust separates• Creates ocean ridges

and rift volcanism

• On land:• Continental Crust

separates• Creates Rift Valleys

and rift volcanism• Ex. African Rift Valley

and Iceland

• Plates move towards each other• Cooler, sinking material pulls plates together

(convection currents)• 3 types of convergent boundaries (classified by

the type of crust involved)

2. Convergent

a) Oceanic-oceanicb) Oceanic-Continentalc) Continental-

Continental

• 2 ocean plates collide• Oceanic plate subducts underneath oceanic plate &

a trench forms• Subducting plate melts forming magma• Magma rises to surface forming a chain of volcanic

islands (island arc)

a) Oceanic-Oceanic Convergence

Subduction – one plate goes

underneath another

Ex. - JAPAN

Why is Japan known for its earthquakes

and tsunamis?

• An ocean plate and a continental plate collide• Oceanic plate subducts underneath continental

plate which forms a trench at coastlines• Subducting plate melts forming magma• Magma rises to surface forming a volcanic

mountain range on the continents

b) Oceanic-Continental Convergence

Ex. - CASCADES

Ocean Crust

denser than

continental crust!

Subduction ALWAYS forms Volcanoes!

• Continental crust collides with continental crust• Crust same density – NO SUBDUCTION• Folded Mountains form as rock gets crumpled

c) Continental-Continental

Ex. – Sierra Nevadas & Himalayas

Old Rocks Young Rocks

Evidence of Moving Plates

• Strike-Slip• 2 plates slide past

each other• Crust deformed or

fractured as giant slabs of crust grind past each other

• Causes earthquakes

• Ex. San Andreas

3. Transform

• Ridge Push – heat rising pushes crust at a ridge toward a trench.

• Slab Pull – cooler material sinking pulls oceanic crust down into a subduction zone (trench)

Ridge Push and Slab Pull